Why is Alternative rock/metal is still called "alternative"?

Look at history, people who created Alternative rock were just bored
from hearing all that Hard Rock/Heavy Metal bands (like Zeppelin,
Rush, Kiss, Ac/Dc etc...) because it stayed popular for 30 years
straight (from the 60's). But ironically, Alternative rock is also
popular for 30 years now. It was created in the 1980's and all the new
rock bands are always "alternative". Why do we keep calling this music
alternative when obviously it's alternative to nothing anymore!?